Adenine-arabinoside (ARA-A) treatment of 11 immunosuppressed patients with varicellazoster infections, as part of a nationwide efficacy trial, appeared to decrease the duration of virus shedding and new vesicle formation. Similar efficacy trials in herpes simplex encephalitis and mucocutaneous herpes simplex infections are currently underway. An in vitro assay system provided a rapid, reliable method to determine ARA-A levels. Guinea pig models of systemic herpes simplex virus infections were established.